Nucleophilic reactivity alpha effect

The effects of the nucleophile on aromatic substitution which are pertinent to our main theme of relative reactivity of azine rings and of ring-positions are brought together here. The influence of a nucleophile on relative positional reactivity can arise from its characteristics alone or from its interaction with the ring or with ring-substituents. The effect of different nucleophiles on the rates of reaction of a single substrate has been discussed in terms of polarizability, basicity, alpha effect (lone-pair on the atom adjacent to the nucleophilic atom), and solvation in several reviews and papers. ... 

Enhanced nucleophilicity of an attacking nucleophile attributable to one or more unshared pairs of electrons on an atom that lies immediately adjacent to the nucleophilic group ". The increased nucleophihcity of hydrazines and hydroxylamines is due to an alpha effect, as is the increased nucleophilicity of H02 compared to HO in solution. This effect on nuclephile reactivity can... 

As is also true for ambident anions, substances exhibiting alpha effects in their reactions consistently deviate from the anticipated structure-reactivity correlations known for simple nucleophiles. 

In order to increase the stability of the reactive carbenium species and modify their reactivity, the effects of attachment of sulfide, pyridine, and phosphine groups on the alpha carbon have been studied (see Section II,D,3). Treatment of [Co2(/l,t, tj -HC2CH2)(CO)6]+ with the nucleophiles (nuc = SR2, PR3, and Py) affords the cationic complexes [Co2(p,rj1,ri2-HC2CH2nuc)(CO)6]+ in high yields, as shown in Eq. (9).79... 

The alpha effect, the enhanced reactivity of nucleophiles with a lone pair of electrons on an adjacent center, has been investigated in reactions with substituted phenyl-phenyl carbonates in water/DMSO (dimethyl sulfoxide) solvent. The enhanced reactivity of oxi-mate ions, which are alpha nucleophiles, was attributed to solvation effects in the ground state rather than to transition state effects. ... 

The most common deviation is the exceptionally high reactivity of nucleophiles, such as hydroperoxide, hypochlorite and hydroxamate ions, with atoms bearing lone-pair electrons next to the nucleophilic centre. This phenomenon, known as the alpha-effect287, is found for aminolysis reactions of esters also285, and is commonly observed for attack at electrophilic centres where reactivity depends fairly strongly on the basicity of the nucleophile. Negative deviations may be evidence of steric hindrance, or in a few cases, in particular that of hydroxide ion, may reflect special solvation effects on the pKa or the nucleophilicity (or both) of the nucleophile. 

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